This invention relates to a heat sealing method and an apparatus for heat sealing thermoplastic materials, such as nonwoven fabric and plastic which are widely used as a medical sanitation material and a packing material.
The method for heat sealing a thermoplastic material (thermal fusion material) is convenient in the points that it does not require any adhesives and it can form a fused portion with a desired shape. Therefore, it is widely used. The well-known heat sealing methods include bar seal method, sliding nip seal method, belt seal method, and rolling roller method which heat seals the sheet continuously by heating up either a roller or an opposing roller and passing a plurality of belt like sheets, at least one of which is made of a thermoplastic material, through a space between the rollers. A heat seal blade is often attached to the roller. In this instance, sealing portions in a variety of shapes are formed on the thermal fusion sheet following the trace formed by rotations of the heat seal blade provided on the peripheral surface of the roller.
Basically, it is possible to heat seal a thermal fusion material, however, insufficient sealing is often formed when multi-layer materials or thick sheet materials are sealed. Normally, the surface of the sheet contacting with the other sheet to be sealed together (sealing surface) has lower temperature than the surface of the sheet contacting with a heat roller. Therefore, it is particularly difficult to raise the temperature of the sealing surfaces of multi-layer materials or thick sheet materials up to the level sufficient for heat sealing in such a short time that heat sealing has completed by contacting these materials to a heat roller. The productivity decreases as a manufacturing speed decreases if only aimed to attain the temperature level sufficient for heat sealing by increasing a contacting time between the heat roller and the sheet. On the other hand, if the temperature of the heat roller is set higher, undesirable phenomenon such as the surfaces of multi-layer materials or thick sheet materials might be ruptured to have holes or becomes stiff likely occur.
An adaptation of a new method is thought, in which the seal blade is applied to the sealing portion a plurality of times while keeping the temperature of the heat roller in the level which does not adversely affect the surface of multi-layer sheets or thick sheets. The results of the examination conducted by inventors for the present invention, however, show that sealing operations following the first sealing are performed on the part deviates from the first sealing portion when the sheet is sealed continuously by a plurality of heat seal apparatuses (which is equipped with a pair of rollers). If such deviations occur among sealing portions, desired sealing strength cannot be attained and the overall appearance becomes worse due to the expansion in the area of sealed parts.
Therefore, the present invention is made to provide a heat sealing method and an apparatus which can heat seal the same part of the multi-layer sheet material or the thick thermal fusion sheet material a plurality of times to obtain sufficient sealing strength without adversely affecting the surface layer of those materials.
According to an aspect of the present invention, a method for heat sealing a plurality of sheets passing through a space between a seal roller and an opposing roller opposingly provided to the seal roller, the method comprises the step of providing a plurality of seal rollers which are provided with seal blades to the opposite roller and step of sealing the same sealing portion of the sheets at least twice. It becomes probable to heat seal the same sealing portion of the sheet a plurality of times by the adoption of this structure. As a result, sealing can be performed within the temperature range which does not adversely affect the surface layer of the sheet and the sufficient sealing strength can be obtained.